These days, worldwide pandemic has been caused by H1N1 influenza virus and occurrence of pandemic by a further virulent virus in the future is a concern.
At present, as therapeutic agents for influenza, e.g., Oseltamivir, Zanamivir, Peramivir, Laninamivir and Amantadine are used. However, these therapeutic agents have, for example, the following drawbacks. Oseltamivir cannot be administered to patients having difficulty in oral administration. It is difficult to administer Zanamivir to children and aged persons. It takes a long time to administer Peramivir. Amantadine is ineffective against Type B influenza virus and resistant viruses have emerged. Laninamivir is an inhalation drug, which is not suitable to patients with dementia and a severe disease and it is difficult to administer it to children.
A further superior therapeutic agent for influenza has been desired. Particularly, an injection that can be administered to patients having difficulty in oral administration, children and aged persons has been desired.
Meanwhile, many reports have been published on methods for improving solubility of medicinal compounds in water. For example, a method for improving the solubility in water by changing the state of a water-insoluble medicinal compound into an amorphous state is known. In general, an amorphous state of a compound exhibits more superior solubility in water than a crystalline state thereof (PATENT DOCUMENT 1).
Compound A or a salt thereof has a superior antiviral activity and is useful as a therapeutic agent for viral infection (PATENT DOCUMENT 2). However, Compound A has low solubility in water and thus an injection of Compound A or a salt thereof has not been known yet.
The present inventors prepared an aqueous solution of a sodium salt of Compound A by using sodium hydroxide generally used as a base, and then produced a lyophilized preparation in accordance with a common method, for improving the water solubility of Compound A. The sodium salt of Compound A obtained by this process was an amorphous dry powder, thus quick dissolution thereof in water was expected. However, contrary to the expectation, the obtained lyophilized preparation was a lyophilized cake having poor solubility and required a long time to dissolve. To describe more specifically, when a solvent was added to the lyophilized preparation, the lyophilized cake changed into a poorly soluble mass substance that took a long time to dissolve. The amorphous lyophilized preparation of a sodium salt of Compound A was a preparation needing a long time to dissolve, and was difficult to handle with significant loss of convenience in use.
Meanwhile, as a method for preparing a lyophilized preparation, a process having an annealing step provided therein is known. However, the effect of the annealing step on the solubility of a lyophilized preparation differs depending on substances. Therefore, studies have been made in a trial-and-error manner (NON-PATENT DOCUMENT 1).